In the design of RF power amplifiers, such as for RF communication applications, it is desirable to improve the efficiency of an amplifier or amplification system while maintaining a desirable system linearity. Linearity and efficiency are often competing design characteristics when configuring a suitable RF amplification system. While linearity is required to reduce interference between adjacent RF signals in a band, to maintain the amplified signals in the band and to reduce distortion, the most linear amplifiers are also usually the most inefficient.
There have been various different amplifier designs utilized to improve efficiency. One design is the Doherty-type or Doherty amplifier, which utilizes a main amplifier or carrier, and an auxiliary or peaking amplifier to handle higher input signal levels. That is, in a certain range of input signal level, generally only the main amplifier is operational and providing the desired signal gain. However, at a higher input signal level, the auxiliary amplifier also begins to operate and contributes to the overall gain of the Doherty amplifier.
Doherty amplifiers offer a method of improving RF/microwave amplifier efficiency over that achieved by traditional Class AB amplifiers. The benefit is achieved when the Doherty amplifier operates at a power level that is backed off from the maximum power achievable at the amplifier output. Some examples of Doherty amplifier designs are set forth in U.S. Pat. No. 6,922,102, entitled “High Efficiency Amplifier” and U.S. patent application Ser. No. 10/795,055, entitled “High Efficiency Amplifier and Method of Designing Same,” both patent and application being incorporated herein by reference in their entireties. Generally, in most applications, the power gain of such a Doherty amplifier is not sufficient to provide the entire gain required by the power amplifier. In such cases, the Doherty amplifier is preceded by at least one additional gain or amplification stage. Typically, the gain stage immediately preceding the Doherty stage is a Class AB device as illustrated in FIG. 1. This choice is generally a compromise between efficiency and linearity, considering the degradation in system efficiency that would occur if the driver stage were a Class A amplifier, and considering the degradation in system linearity that would occur if the driver stage were also a Doherty amplifier.
However, a driver stage that is also a Doherty amplifier offers better overall system efficiency, even though overall system linearity might be degraded when compared to using a Class A and Class AB driver stage. The existing design as shown in FIG. 1 does not achieve the desired efficiency. Therefore, it is still desirable to achieve better overall system efficiency by improving the driver stage efficiency without suffering a significant loss in system linearity. The present invention, as discussed further herein below, achieves these desirable characteristics and other goals as noted herein.